Integrated devices are normally prepared on substrates in the form of wafers, which act mainly as support in their manufacture. However, the increase in the degree of integration and in the performances expected for these devices is leading to an increasingly significant coupling between their performances and the characteristics of the substrate on which they are formed. This is particularly the case with radio frequency (RF) devices, which process signals for which the frequency is between approximately 3 kHz and 300 GHz, which in particular have application in the field of telecommunications (telephony, Wi-Fi, BLUETOOTH®, and the like).
As example of the device/substrate coupling, electromagnetic fields, which result from the high frequency signals propagating in the devices, penetrate into the depth of the substrate and interact with the possible charge carriers that are found therein. This results in an unnecessary consumption of a portion of the energy of the signal by insertion loss and possible influences between components by crosstalk.
Radio frequency devices, such as switches and aerial tuners, and also power amplifiers, can be prepared on substrates specifically adapted in order to take into account these phenomena and in order to improve the performances thereof.
Thus, silicon-on-insulator (SOI) substrates are known that comprise, as is represented in FIG. 1, a support substrate 2, a charge-trapping layer 3 positioned on the support substrate 2, an insulating layer 4 positioned on the charge-trapping layer 3, and a surface silicon layer 5 positioned on the insulating layer. The support substrate 2 can exhibit a resistivity of greater than 1 kohm·cm. The charge-trapping layer 3 can comprise nondoped polycrystalline silicon. The charge-trapping layer 3 makes it possible to reduce the abovementioned device/substrate coupling and thus to ensure good performances for the RF devices. The manufacture of this type of substrate is described, for example, in the documents FR 2 860 341, FR 2 933 233, FR 2 953 640 and US 2015/115480.
The Applicant Company has observed that the application of a rapid heat treatment applied to such an SOI substrate could result in the radiofrequency properties of this substrate being affected. These rapid heat treatments are of particular use in treating the surface of a substrate during its manufacture. This is also a necessary stage in the normal processes for the manufacture of CMOS components, for example for the activation of the dopants.
More specifically, the Applicant Company has observed that the “second harmonic distortion” characterization measurement proves to be of the order of 25% lower in the case of a substrate, which has been subjected to a rapid heat treatment, than in the case of a substrate that has not been subjected to a rapid heat treatment.
This characterization measurement, a detailed description of which will be found in the document entitled “White paper—RF SOI Characterisation” and published by Soitec in January, 2015, is particularly relevant as it is very representative of the expected performance of an RF device formed on the substrate characterized.
However, the level achieved by this measurement on the SOI substrates exhibiting a charge-trapping layer and which have been subjected to a rapid heat treatment is not sufficiently high to guarantee the operation of the RF device under the specifications required.